Copolymer compound and electrochemical cell therewith

ABSTRACT

The present invention relates to a copolymer compound prepared by copolymerizing two or more monomers selected from indole and indole derivatives. The copolymer compound can be used as an electrode active material to provide an electrochemical cell with an increased capacity and improved cycle properties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a conductive compound used as an electrode active material in an electrochemical cell represented by a secondary battery or an electric double-layer capacitor, as well as an electrochemical cell with the conductive compound. Particularly, it relates to a polymer compound whereby an appearance capacity can be improved without deterioration in cycle properties, as well as an electrochemical cell therewith.

2. Description of the Related Art

There have been suggested and practically used electrochemical cells such as secondary batteries and electric double-layer capacitors in which a proton-conducting compound is used as an electrode active material. FIG. 4 shows a schematic cross-sectional view of an example of a primitive cell 1 constituting an electrochemical cell according to the prior art.

As shown in FIG. 4, a conventional electrochemical cell has a configuration where a cathode 2 comprising a proton-conducting compound as an electrode active material and an anode 3 comprising a proton-conducting compound as an electrode active material are formed on a cathodic collector 4 and an anodic collector 5, respectively and these electrodes are laminated via a separator 6 and which is filled with an aqueous or non-aqueous solution containing a proton source and is sealed by a gasket 7. Operation of the electrochemical cell involves only protons as a charge carrier.

A cathode 2 and an anode 3 are formed using an electrode material comprising a doped or undoped proton-conducting compound powder, a conductive auxiliary and when necessary a binder. An electrode may be formed by an appropriate method such as pressure forming of an electrode material and depositing an electrode material slurry on a conductive substrate. The cathode 2 and the anode 3 thus formed can be mutually faced via a separator 6 to give a primitive cell 1.

Examples of a proton-conducting compound used as an electrode active material include π-conjugated polymers such as polyaniline, polythiophene, polypyrrole, polyacetylene, poly-p-phenylene, polyphenylene-vinylene, polyperinaphthalene, polyfuran, polyflurane, polythienylene, polypyridinediyl, polyisothianaphthene, polyquinoxaline, polypyridine, polypyrimidine, polyindole, polyaminoanthraquinone, polyimidazole and their derivatives; indole π-conjugated compounds such as an indole trimer; quinones such as benzoquinone, naphthoquinone and anthraquinone; quinone polymers such as polyanthraquinone, polynaphthoquinone and polybenzoquinone where a quinone oxygen can be converted into a hydroxyl group by conjugation; and a copolymer from two or more monomer units constituting the above polymers. These compounds may be doped to form a redox pair for exhibiting conductivity. These compounds are appropriately selected as a cathode and an anode active materials, taking a redox potential difference into account.

Known electrolytic solutions include an aqueous electrolytic solution consisting of an aqueous acid solution and a non-aqueous electrolytic solution comprising an electrolyte in an organic solvent. When using a proton-conducting compound, the former aqueous electrolytic solution is mostly used because it can give a high-capacity cell. The acid used may be an organic or inorganic acid; for example, inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, tetrafluoroboric acid, hexafluorophosphoric acid and hexafluorosilicic acid and organic acids such as saturated monocarboxylic acids, aliphatic carboxylic acids, oxycarboxylic acids, p-toluenesulfonic acid, polyvinylsulfonic acid and lauric acid.

Japanese Patent Application publication Nos. 2002-93419A, 2003-142099A and 2003-249221A have disclosed a trimerized indole or indole derivative (indole trimer) as a compound used as the above electrode active material. These patent references have a secondary battery and a capacitor comprising as an electrode active material an indole trimer having a fused cyclic structure at 2- and 3-positions of a substituted indoles.

Formula (4) shows a reaction mechanism of charge/discharge for a substituted indole trimer in a electrolytic solution containing a proton source. In formula (4), R represents a given substituent; X⁻ represents a given anion; and a moiety encircled with a dashed line represents a redox inert site.

As seen from the formula, this compound has three redox active sites, but only two of these are utilized in a redox reaction in a practical electrochemical cell. Therefore, a capacity obtained is only two thirds of a theoretical capacity estimated from a monomer. Thus, a capacity density per a unit weight of the substituted indole trimer is reduced, resulting in a reduced appearance capacity of an electrochemical cell comprising the trimer as an electrode active material.

Furthermore, in a substituted indole trimer, repeated doping and dedoping in association with charge/discharge causes a crystal structure to be altered, leading to increase in an internal resistance of the electrode and thus deterioration in cycle properties.

SUMMARY OF THE INVENTION

An objective of the present invention is to increase a capacity per a unit weight of an electrode active material in an electrochemical cell comprising a compound derived from an indole derivative as the electrode active material, for improving cycle properties.

In order to achieve the above objective, the present invention is characterized in that a compound prepared by reacting two or more of indole derivatives having different substituents or substitution positions is used as an electrode active material.

According to an aspect of this invention, there is provided a copolymer compound prepared by copolymerizing two or more monomers selected from indole and indole derivatives represented by formula (1):

wherein Rs independently represent hydrogen atom, nitro group, carboxyl group, carboxylate group, cyano group, acetyl group, aldehyde group and halogen atom.

According to another aspect of this invention, there is provided the copolymer compound as described above, comprising a unit represented by formula (1A) and a unit represented by formula (1B) or (1C):

wherein Rs independently represent hydrogen atom, nitro group, carboxyl group, carboxylate group, cyano group, acetyl group, aldehyde group and halogen atom.

According to another aspect of this invention, there is provided the copolymer compound as described above, represented by formula (2):

wherein Rs independently represent hydrogen atom, nitro group, carboxyl group, carboxylate group, cyano group, acetyl group, aldehyde group and halogen atom; and n represents a natural number.

According to another aspect of this invention, there is provided the copolymer compound as described above, wherein the monomer comprises an indole derivative having a substituent other than hydrogen at least at 3-position.

According to another aspect of this invention, there is provided the copolymer compound as described above, represented by formula (3):

wherein Rs independently represent hydrogen atom, nitro group, carboxyl group, carboxylate group, cyano group, acetyl group, aldehyde group and halogen atom; and n represents a natural number.

According to another aspect of this invention, there is provided the copolymer compound as described above, wherein the monomer comprises an indole derivative having a substituent other than hydrogen at least at 2-position.

According to another aspect of this invention, there is provided the copolymer compound as described above, which is a proton-conducting compound initiating an electrochemical redox reaction in a solution containing a proton source.

According to another aspect of this invention, there is provided an electrochemical cell comprising at least one selected from the copolymer compounds described above as an electrode active material.

According to another aspect of this invention, there is provided an electrochemical cell comprising at least one selected from the copolymer compounds described above amounting to 10 to 100% by weight to the total amount of electrode active materials in a relevant electrode.

According to another aspect of this invention, there is provided an electrochemical cell comprising at least one selected from the copolymer compounds described above as an electrode active material in a cathode.

According to another aspect of this invention, there is provided an electrochemical cell comprising, as an electrode active material in a cathode, at least one selected from the copolymer compounds described above amounting to 10 to 100% by weight to the total amount of electrode active materials in the cathode.

According to another aspect of this invention, there is provided the electrochemical cell described above comprising an electrolyte containing a proton source, wherein protons act as a charge carrier in a redox reaction in association with charge/discharge.

The copolymer compound of the present invention is an oligomer or polymer compound which has a different chemical structure from a conventional substituted indole trimer having a fused cyclic structure at 2- or 3-position, and a main chain composed of a substituted indole.

A first effect of such a structure is that when used as an electrode active material in an electrochemical cell, it allows a redox active site to be effectively utilized. A second effect is that a morphology of the electrode active material surface is altered to promote doping or dedoping of ions in an electrolytic solution, resulting in improvement in a charge/discharge efficiency. A third effect is that an electrode active material can be made amorphous to prevent reduction in electron conductivity due to destruction of a crystal structure caused by doping or dedoping in association with charge/discharge.

Thus, in an electrochemical cell in which an electrode material comprises the above copolymer compound as an electrode active material, a capacity per a unit weight of the electrode active material is increased, resulting in increase in an appearance capacity. Furthermore, increase in an internal resistance can be prevented, resulting in improved cycle properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b show SEM images of copolymer compounds in Example and Comparative Example, respectively.

FIG. 2 is a graph showing the evaluation results for TG for a copolymer compound.

FIG. 3 is a graph showing the measurement results of a polymerization potential.

FIG. 4 shows a schematic cross-sectional view of an example of a primitive cell in an electrochemical cell according to the prior art.

DESCRIPTION OF PREFERRED EMBODIMENTS

A copolymer compound of the present invention is suitable as an electrode material in an electrochemical cell comprising a proton-conducting compound as an electrode active material. As an electrode active material, preferably as a cathode active material, a copolymer compound prepared from two or more of compounds selected from the indole and its derivatives represented by formula (1) can be used to provide an electrochemical cell improved in an appearance capacity and cycle properties.

Such a copolymer compound may be preferably a copolymer compound having a unit represented by formula (1A) and a unit represented by formula (1B) or (1C), including a copolymer compound represented by formula (2) or (3). In these formulas, an alkyl moiety in a carboxylate group may be alkyl having 1 to 8 carbon atoms and halogen may be selected from fluorine, chlorine, bromine and iodine.

A copolymer composition ratio of the copolymer compound, when defined as a molar ratio of the unit of formula (1A) to the unit of formula (1B) and/or (1C) (i.e., ((1A):(1B) and/or (1C)), is preferably 2:1 to 1:5, more preferably 1:0.9 to 1:5, further preferably 1:1 to 1:5 in the light of improvement in an appearance capacity and copolymerization properties. Particularly, a 1:1 alternating copolymer is suitably used as an electrode active material.

The copolymer compound with a weight average molecular weight of 2,000 to 20,000, preferably 2,000 to 10,000, more preferably 3,000 to 6,000 may be suitably used as an electrode active material. The average molecular weight can be determined by gel permeation chromatography (GPC) using polystyrene as a standard sample.

Starting materials for preparing a copolymer compound of the present invention may be appropriately selected from indole and indole derivatives. Specific examples include indole, mono-substituted, di-substituted and tri-substituted monomers such as 2-nitroindole, 3-nitroindole, 4-nitroindole, 5-nitroindole, 6-nitroindole, 7-nitroindole, indole-2-carboxylic acid, indole-3-carboxylic acid, indole-4-carboxylic acid, indole-5-carboxylic acid, indole-6-carboxylic acid, indole-7-carboxylic acid, methyl indole-2-carboxylate, methyl indole-3-carboxylate, methyl indole-4-carboxylate, methyl indole-5-carboxylate, methyl indole-6-carboxylate, methyl indole-7-carboxylate, ethyl indole-2-carboxylate, ethyl indole-3-carboxylate, ethyl indole-4-carboxylate, ethyl indole-5-carboxylate, ethyl indole-6-carboxylate, ethyl indole-7-carboxylate, 2-cyanoindole, 3-cyanoindole, 4-cyanoindole, 5-cyanoindole, 6-cyanoindole, 7-cyanoindole, 2-acetylindole, 3-acetylindole, 4-acetylindole, 5-acetylindole, 6-acetylindole, 7-acetylindole, indole-2-aldehyde, indole-3-aldehyde, indole-4-aldehyde, indole-5-aldehyde, indole-6-aldehyde, indole-7-aldehyde, 2-bromoindole, 3-bromoindole, 4-bromoindole, 5-bromoindole, 6-bromoindole, 7-bromoindole, indole-2,6-dicarboxylic acid, indole-3,6-dicarboxylic acid, indole-4,5-dicarboxylic acid, indole-4,6-dicarboxylic acid, indole-5,6-dicarboxylic acid, methyl indole-2,6-dicarboxylate, methyl indole-3,6-dicarboxylate, methyl indole-4,5-dicarboxylate, methyl indole-4,6-dicarboxylate, methyl indole-5,6-dicarboxylate, ethyl indole-2,6-dicarboxylate, ethyl indole-3,6-dicarboxylate, ethyl indole-4,5-dicarboxylate, ethyl indole-4,6-dicarboxylate, ethyl indole-5,6-dicarboxylate, 2,6-diacetylindole, 3,6-diacetylindole, 4,5-diacetylindole, 4,6-diacetylindole, 5,6-diacetylindole, methyl 2-acetylindole-6-carboxylate, methyl 3-acetylindole-6-carboxylate, methyl 2-acetylindole-5,6-dicarboxylate, and methyl 3-acetylindole-5,6-dicarboxylate. Two or more appropriately selected from these monomers may be subjected to polymerization to give a copolymer compound.

There will be described a process for preparing a copolymer compound of the present invention and a manufacturing process for an electrochemical cell. First, an electrolytic polymerization method will be described.

A copolymer compound can be prepared by dissolving an electrolyte such as lithium tetrafluoroborate to a concentration of about 0.3 mol/L in a solvent such as acetonitrile, adding two or more of indoles having a given substituent as monomers to the solution and applying a voltage under a potential sweep range of 500 to 1600 mV and a potential sweep rate of 50 mV/s using a potentiostat. A product deposited on a working electrode is washed with an appropriate solvent to give a powdery or film solid.

Other compounds may be used as the electrolyte without being limited to lithium tetrafluoroborate described above. Examples include perchloric acid, lithium perchlorate, sodium perchlorate, tetrabutylammonium perchlorate, tetraethylammonium perchlorate, tetraethylammonium tetrafluoroborate and tetrabutylammonium tetrafluoroborate. A reaction period of the electrolytic polymerization is preferably, but not limited to, 0.1 to 10 hours in the light of preventing side reactions.

A copolymer compound may be prepared by any proper method such as a chemical oxidative polymerization without being limited to the electrolytic polymerization described above. The copolymer can be prepared by dissolving two or more indoles having a given substituent as monomers in a polymerization solvent such as acetonitrile, adding an oxidizing agent such as ferric chloride to the solution, and stirring the mixture. A deposited product can be filtrated and washed with an appropriate solvent to give the copolymer compound.

Herein, other solvents and oxidizing agents may be used as a polymerization solvent and an oxidizing agent, without being limited to acetonitrile and ferric chloride described above. Examples of a polymerization solvent include aromatic hydrocarbons such as toluene, xylenes and chlorobenzene; halogenated aliphatic hydrocarbons such as dichloromethane and chloroform; acetates such as methyl acetate, ethyl acetate and butyl acetate; aprotic polar solvents such as dimethylformamide, dimethylacetamide, N-methylpyrrolidone, tetramethylurea, hexamethylphosphoric triamide (HMPA); ethers such as diethyl ether, tetrahydrofuran and dioxane; aliphatic hydrocarbons such as pentane and n-hexane; aliphatic alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, s-butanol and t- butanol; acetone; acetonitrile; and propionitrile, preferably acetone, acetonitrile, dioxane and dimethylformamide. They may be used alone or in combination of two or more in an appropriate mixing rate.

Examples of an oxidizing agent include ferric chloride hexahydrate, anhydrous ferric chloride, ferric nitrate nonahydrate, ferric nitrate, ferric sulfate n-hydrate, ferric ammonium sulfate dodecahydrate, ferric perchlorate n-hydrate, ferric tetrafluoroborate, cupric chloride, cupric sulfate, cupric tetrafluoroborate, tetrafluoroborate nitrosonium, ammonium persulfate, sodium persulfate, potassium persulfate, sodium periodate, potassium periodate, hydrogen peroxide, ozone, potassium hexacyanoferrate, tetraammonium cerium (IV) sulfate dihydrate, bromine and iodine, preferably ferric chloride hexahydrate, anhydrous ferric chloride, ferric nitrate nonahydrate, ferric nitrate, ferric sulfate n-hydrate, ferric ammonium sulfate dodecahydrate, ferric perchlorate n-hydrate and ferric tetrafluoroborate. These oxidizing agents may be used alone or in combination of two or more in an appropriate mixing rate.

In the chemical oxidative polymerization, a reaction temperature may be within a range of 0° C. to a refluxing temperature of a solvent used, preferably 10 to 100° C. A reaction period of the chemical oxidative polymerization is preferably, but not limited to, 0.1 to 100 hours in the light of preventing side reactions.

In the electrolytic polymerization, a polymerization potential is measured with a potential sweep range of 500 mV to 1600 mV and a potential sweep rate of 50 mV/s and can be used as a measure for determining possibility of copolymerization. Furthermore, a copolymer compound thus prepared is subjected to scanning electron microscopy (hereinafter, referred to as “SEM”) and thermogravimetry (hereinafter, referred to as “TG”) and the results are used as means for determining formation of a copolymer compound, which is different from compounds in the prior art.

Next, there will be described a preparation method for a test sample and test conditions for cyclic voltammetry (hereinafter, referred to as “CV measurement”). To a copolymer compound formed as a conductive auxiliary is added VGCF® from Showa Denko K.K. (hereinafter, referred to as “VGCF”), a fibrous carbon by a vapor growth method to an amount of 30% by weight to the copolymer compound. The mixture is mixed and applied to a carbon fiber sheet (Toray Industries, Inc., TGP-H-030). The sheet is dried at 120° C. to prepare a measurement sample.

CV measurement is conducted using a 20 wt % sulfuric acid as an electrolytic solution under a potential sweep range of 200 to 1200 mV and a potential sweep rate of 20 mV/s. An integrated CV capacity in the potential sweep range of 200 to 1200 mV is calculated and normalized per a unit weight of the copolymer compound.

Next, there will be described a configuration of an electrochemical cell and a manufacturing process therefor.

The present invention is characterized by a copolymer compound used as an electrode active material, and a structure of a primitive cell may be as in the prior art shown in FIG. 4. It will be, therefore, described with reference to FIG. 4.

An electrochemical cell of the present invention is a proton-conducting electrochemical cell where protons act as a charge carrier in a redox reaction in association with charge/discharge. More specifically, it comprises an electrolyte containing a proton source and preferably operates such that adsorption/desorption of protons of an electrode active material is exclusively involved in electron transfer in a redox reaction in association with charge/discharge.

Such an electrochemical cell may comprise respective proton-conducting compounds as cathode and anode active materials, and comprise an electrolytic solution containing a proton source as an electrolyte.

A proton-conducting compound used as an electrode active material may be, in addition to a copolymer compound of this present invention, any of those known compounds without limitation as long as it can initiate a redox reaction in absolution containing a proton source.

Examples of a proton-conducting compound used as an electrode active material include π-conjugated polymers such as polyaniline, polythiophene, polypyrrole, polyacetylene, poly-p-phenylene, polyphenylene-vinylene, polyperinaphthalene, polyfuran, polyflurane, polythienylene, polypyridinediyl, polyisothianaphthene, polyquinoxaline, polypyridine, polypyrimidine, polyindole, polyaminoanthraquinone, polyimidazole and their derivatives; indole π-conjugated compounds such as an indole trimer compound; quinones such as benzoquinone, naphthoquinone and anthraquinone; quinone polymers such as polyanthraquinone, polynaphthoquinone and polybenzoquinone where a quinone oxygen can be converted into a hydroxyl group by conjugation; and copolymers prepared from two or more of the monomers giving the above polymers. These compounds may be doped to form a redox pair for exhibiting conductivity. These compounds may be appropriately selected as a cathode and an anode active materials, taking a redox potential difference into account.

An electrode in an electrochemical cell of the present invention is characterized in that it comprises a proton-conducting compound as an electrode active material, a conductive auxiliary and when necessary a binder, and comprises a copolymer compound prepared from two or more indole derivative monomers as an electrode active material, preferably a cathode active material.

A proportion of a copolymer compound of the present invention in the total amount of active materials in the electrode is preferably 10 to 100 wt %, more preferably 20 to 100 wt %, further preferably 30 to 100 wt % in the light of achieving desired effects.

A cathode 2 may comprise, for example, a copolymer compound of the present invention as an electrode active material preferably in an amount of 10 to 100 wt %. A conductive auxiliary such as VGCF may be added in an amount of 1 to 50 wt %, preferably 10 to 30 wt % to the amount of a cathode active material. A binder such as polyvinylidene fluoride (hereinafter, referred to as “PVDF”) may be added to and mixed with a cathode active material in an amount of 1 to 20 wt %, preferably 5 to 10 wt % to the amount of the active material. The resulting mixed powder is pressed at 0 to 300° C., preferably 100 to 250° C., to give the cathode 2.

An anode 3 can be formed by pressing and firing a powdery mixture of an electrode active material such as polyphenylquinoxaline, a conductive auxiliary such as Ketjen Black™ (Product name: Ketjen Black EC-600JD, from Ketjen Black International) in a weight ratio of 72:28.

An electrolytic solution may be a proton-containing aqueous or non-aqueous solution. An aqueous electrolytic solution is preferable because it can provide a high-capacity cell. An acid used may be an organic or inorganic acid, including inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, tetrafluoroboric acid, hexafluorophosphoric acid and hexafluorosilicic acid and organic acids such as saturated monocarboxylic acids, aliphatic carboxylic acids, oxycarboxylic acids, p-toluenesulfonic acid, polyvinylsulfonic acid and lauric acid.

A content of protons in an electrolytic solution is preferably 10⁻³ mol/L to 18 mol/L, more preferably 10⁻¹ mol/L to 7 mol/L. A too low concentration may result in insufficient performance as an electrolytic solution while a too high concentration may give a strongly acidic solution, leading to deterioration in material activity and dissolution of the materials.

A separator 6 may be a polyolefin porous film or cation-exchange film with a thickness of 10 to 50 μm. A cathodic current collector 4 or an anodic current collector 5 may be a rubber sheet which is made conductive by dispersed conductive carbon powder. A gasket 7 may be, for example, an butyl-rubber.

The members described above may be combined to provide a primitive cell 1. Specifically, as shown in FIG. 1, on a cathodic current collector 4 and an anodic current collector 5 are disposed a cathode 2 and an anode 3, respectively, which are then laminated via a separator 6. The cell is filled with an electrolytic solution and sealed by a gasket 7. The cell here has a coin type of external shape, but may have any commonly used shape such as a laminate type and a roll type without limitation.

EXAMPLES

This invention will be more specifically with reference to Examples.

Example 1

Methyl indole-6-carboxylate and 3-acetylindole were selected as substituted indole monomers. In acetonitrile as a polymerization solvent were dissolved these monomers to 20×10⁻³ mol/L and lithium tetrafluoroborate as an electrolyte to 0.3 mol/L. The mixture was subjected to electrolytic polymerization using a potentiostat. Precipitation of a product on a working electrode was observed. The product was rinsed with ethanol and dried to give a dark green copolymer compound powder.

The copolymer compound thus obtained was analyzed. First, its SEM photo image was taken. Here, an SEM photo image of a trimer having a fused cyclic structure prepared by connecting 2- and 3-positions of methyl indole-6-carboxylates was taken as Comparative Example. FIGS. 1A and 1B show SEM photo images in Example and Comparative Example, respectively. In the SEM image of Example, a fibrous morphology was observed and the polymerization product had a morphology different from that of Comparative Example.

Then, the copolymer compound prepared was evaluated by TG. The trimer of methyl indole-6-carboxylate was also evaluated. FIG. 2 shows the TG evaluation results for the copolymer compound. The results demonstrate that the copolymer compound of Example did not have a clear decomposition point, indicating improvement in heat resistance. On the other hand, the trimer of methyl indole-6-carboxylate of Comparative Example experienced rapid weight reduction around at 300° C., indicating that it is less heat-resistant.

Next, the copolymer compound prepared above was used as a cathode active material to prepare an electrochemical cell. Here, a copolymer compound, VGCF and PVDF were weighed and blended in a weight ratio of 69/23/8. The mixture was pressed at 200° C. to form a cathode. An anode was formed by weighing and blended polyphenylquinoxaline and Ketjen Black as an electrode active material in a weight ratio of 72/28, pressing the mixture at 300° C. and firing the product.

An electrolytic solution was a 20 wt % aqueous sulfuric acid solution, a separator was a cation-exchange membrane with a thickness of 15 μm, a gasket was a butyl-rubber, and a current collector was a conductive rubber sheet.

These members were used to manufacture an electrochemical cell consisting of the primitive cells shown in FIG. 1.

Example 2

A-copolymer compound was prepared and an electrochemical cell was formed as described in Example 1, except that methyl indole-6-carboxylate and indole-2-aldehyde were used as substituted indole monomers.

Example 3

A copolymer compound was prepared and an electrochemical cell was formed as described in Example 1, except that 5-cyanoindole and indole-5-carboxylic acid were used as substituted indole monomers and 5-cyanoindole and indole-5-carboxylic acid were dissolved in acetonitrile to 50×10⁻³ mol/L and 25×10⁻³ mol/L, respectively, to prepare a polymerization solution.

Example 4

A copolymer compound was prepared and an electrochemical cell was formed as described in Example 1, except that methyl indole-6-carboxylate, 3-acetylindole and indole-3-aldehyde as substituted indole monomers were dissolved in acetonitrile to 40×10⁻³ mol/L, 20×10⁻³ mol/L and 20×10⁻³ mol/L, respectively, to prepare a polymerization solution.

Example 5

Methyl indole-6-carboxylate and 3-acetylindole were selected as substituted indole monomers. They were dissolved in acetonitrile as a polymerization solvent and were subjected to chemical oxidative polymerization. Using ammonium persulfate as an oxidizing agent, the mixture was reacted by stirring at 60° C. for 3 hours. The precipitate was rinsed with ethanol and dried to give a dark green copolymer compound powder. An electrochemical cell was formed as described in Example 1, except that the copolymer compound was used as a cathode active material.

Example 6

A copolymer compound was prepared and an electrochemical cell was formed as described in Example 1, except that methyl indole-6-carboxylate and methyl 3-acetylindole-6-carboxylate were used as substituted indole monomers.

Example 7

A copolymer compound was prepared and an electrochemical cell was formed as described in Example 1, except that methyl indole-5,6-dicarboxylate and methyl 3-acetylindole-6-carboxylate were used as substituted indole monomers.

Example 8

An electrochemical cell was formed as described in Example 1, except that a cathode active material was a mixed powder of the copolymer compound described in Example 1 and a trimer having a fused cyclic structure prepared by connecting methyl indole-6-carboxylates at 2- and 3-positions in a weight ratio of 50/50 and a cathode was an electrode by weighing and blending the electrode active material, VGCF and PVDF in a weight ratio of 69/23/8 and pressed the mixture at 200° C.

Example 9

An electrochemical cell was formed as described in Example 1, except that a cathode active material was a mixed powder of the copolymer compound described in Example 1 and a trimer having a fused cyclic structure prepared by connecting methyl indole-6-carboxylates at 2- and 3-positions in a weight ratio of 20/80 and a cathode was an electrode by weighing and blending the electrode active material, VGCF and PVDF in a weight ratio of 69/23/8 and pressed the mixture at 200° C.

Comparative Example

A trimer having a fused cyclic structure prepared by connecting 2- and 3-positions of 6-methylindoles (6-methylindole trimer) was prepared and used as a cathode active material to form an electrochemical cell. The electrochemical cell was formed as described in Example 1, except that the 6-methylindole trimer, VGCF and PVDF were weighed and blended in a weight ratio of 69/23/8 and the mixture was pressed at 200° C. to give a cathode.

In electrolytic polymerization in the above Examples and Comparative Example, a polymerization potential was determined under the conditions described above. All the copolymer compounds in Examples and Comparative Example were subjected to CV determination under the conditions described above. For the electrochemical cells, an initial capacity and a capacity residual ratio after 5,000 charge/discharge cycles were determined.

FIG. 3 shows the measurement results of a polymerization potential. Table 1 shows initial capacities and capacity residual ratios after 1,000 cycles. Table 2 shows initial capacities and capacity residual ratios after 5,000 cycles for the electrochemical cells. TABLE 1 Capacity residual ratio after Capacity [C/g] 1,000 cycles [%] Increasing Increasing rate Measured rate to Comp. Measured to Comp. Ex. value Ex. [%] value [%] Ex. 1 297 27 94 9 2 284 21 93 8 3 260 11 92 7 4 281 20 91 6 5 295 26 97 9 6 260 11 92 7 7 248 6 91 6 8 265 13 92 11 9 251 7 91 7 Comp. 234 — 85 — Ex.

TABLE 2 Capacity residual ratio after Initial capacity [mAh/g] 5,000 cycles [%] Increasing Increasing rate Measured rate to Comp. Measured to Comp. Ex. value Ex. [%] value [%] Ex. 1 55 51 94 50 2 56 43 88 44 3 50 28 82 38 4 55 41 69 25 5 57 46 90 46 6 47 20 79 35 7 43 10 62 18 8 52 33 85 41 9 45 15 74 30 Comp. 39 — 44 — Ex.

The results in Table 1 demonstrate that the copolymer compounds of Examples had a higher capacity per a unit weight of an electrode active material by at least 6% and a higher capacity residual ratio after 1,000 cycles, i. e., a cycle property, by at least 6% than that in Comparative Example, indicating effectiveness of the present invention. The results in Table 2 demonstrate that the electrochemical cells in Examples have a higher capacity by at least 10% and an improved cycle property by at least 18% than that in Comparative Example.

As described above, an electrode comprising a copolymer compound of the present invention as an electrode active material can be used to provide an electrochemical cell with an increased appearance capacity and improved cycle properties. It is because using a copolymer compound of monomers selected from indole and indole derivatives as electrode materials allows a redox active site to be effectively used; an altered surface morphology results in smooth doping and dedoping to improve a charge/discharge efficiency; and an amorphous electrode active material with an increased molecular weight can prevent materials from being deteriorated due to repeated doping/dedoping in association with charge/discharge. 

1. A copolymer compound comprising copolymerized units of two or more monomers selected from indole and indole derivatives represented by formula (1):

wherein Rs each independently represent hydrogen atom, nitro group, carboxyl group, carboxylate group, cyano group, acetyl group, aldehyde group, or halogen atom.
 2. The copolymer compound as claimed in claim 1, wherein the units comprise a unit represented by formula (1A) and a unit represented by formula (1B) or (1C):

wherein Rs each independently represent hydrogen atom, nitro group, carboxyl group, carboxylate group, cyano group, acetyl group, aldehyde group, or halogen atom.
 3. The copolymer compound as claimed in claim 1, having a structure of formula (2):

wherein Rs each independently represent hydrogen atom, nitro group, carboxyl group, carboxylate group, cyano group, acetyl group, aldehyde group or halogen atom; and n represents a natural number.
 4. The copolymer compound as claimed in claim 3, wherein the monomer comprises an indole derivative having a substituent other than hydrogen at least at 3-position.
 5. The copolymer compound as claimed in claim 1, having a structure of formula (3):

wherein Rs each independently represent hydrogen atom, nitro group, carboxyl group, carboxylate group, cyano group, acetyl group, aldehyde group, or halogen atom; and n represents a natural number.
 6. The copolymer compound as claimed in claim 5, wherein the monomer comprises an indole derivative having a substituent other than hydrogen at least at 2-position.
 7. The copolymer compound as claimed in claim 1, which is a proton-conducting compound capable of initiating an electrochemical redox reaction in a solution containing a proton source.
 8. An electrochemical cell comprising at least one selected from the copolymer compounds as claimed in claim 1 as an electrode active material.
 9. An electrochemical cell comprising at least copolymer compounds as claimed in claim 1 as an electrode active material accounting for 10 to 100-% by weight of the total amount of electrode active materials in a relevant electrode.
 10. An electrochemical cell comprising at least the copolymer compounds as claimed in claim 1 as an electrode active material in a cathode.
 11. An electrochemical cell comprising, as an electrode active material in a cathode, at least the copolymer compounds as claimed in claim 1 accounting for 10 to 100-% by weight of the total amount of electrode active materials in the cathode.
 12. The electrochemical cell as claimed in claim 8 comprising an electrolyte containing a proton source, wherein protons act as a charge carrier in a redox reaction in association with charge/discharge. 